Signatures are digital values that are computed from a key and the information being signed. When you sign a document with your private key, anyone can use your public key to decrypt the signature. This proves the document is from you. It is how PKI provides authentication. If you signed a second document with the same private key, the second signature would be different, which means a would-be forger cannot simply copy a signature from one document to another.

In organizations or situations where security is a major concern and many documents are signed and/or encrypted, the key pair used for signing is different from the key pair used for encrypting. This is because a key pair used for signing may need to have a very long lifetime compared with a key pair used for encryption.

Certificates

Certificates are a kind of digital ID card that use your public key instead of your photo. Being sure of the other party's identity is just as important online as it is in traditional transactions. The idea behind certificates is that if you trust the identity, honesty, and procedures of certificate issuer "Jean," and "Jean" vouches for the identity of "Chris," you can trust that "Chris" is really "Chris" and not an impersonator. In this example, Jean is acting as a certificate authority (CA).

In addition to a public key, the certificate includes other information to identify its owner, as well as a timestamp that says when the certificate will expire. In the example above, the certificate would be signed by Jean using Jean's private key. Decrypting the certificate's signature with Jean's public key would prove that it came from Jean, and thus authenticate Chris' public key. Anyone who trusts Jean can also trust that the public key belongs to Chris, not someone else.

A Certificate Management System (CMS) is used by a CA to issue and manage certificates. Certificates may be revoked for administrative reasons, or perhaps because the private key associated with the public key in the certificate has been compromised. In this example, Jean would invalidate Chris' certificate if Chris' private key was compromised in some way. Jean's CMS places Chris' certificate on a Certificate Revocation List (CRL) when Jean revokes Chris' certificate.

The two most critical things done by a CA are protecting its private key and creating and following a comprehensive, documented procedure for validating the information in the certificates it issues. If the CA's private key were compromised, nobody could trust it or the certificates purportedly signed by it. Just as important, Jean must make sure that Chris really is Chris. This is a matter outside the realm of cryptography and IT. Depending on its thoroughness, the validation procedure can be costly. A CA may issue different grades of certificates, corresponding to different procedures used to validate identity.